Board Thread:Pokémon Help/@comment-28882506-20161204173245/@comment-27248965-20161204174528

Max out its happiness by running around with it in your party. Levelling up and item usage also helps, but I recommend that you should evolve it at Lv.13 so that it learns Electro Ball when evolving into Pikachu. Most importantly, do not let it faint.

You can check your Pichu's happiness by having it lead your party and head to Brimber City for the Happiness checker. If he says that your Pichu "can't possibly love you more", the next time it levels up, it is guaranteed to evolve.